1. Field
Embodiments of the present invention relate to a sealing member for a master cylinder and a master cylinder having the same, in which a communication hole of a piston is rapidly brought into communication with an oil path toward an oil tank upon release of braking, so as to minimize delay in a braking release operation.
2. Description of the Related Art
A master cylinder is a device to generate hydraulic pressure in a hydraulic brake system. FIG. 1 illustrates an example of a conventional tandem master cylinder and oil tank for use in an automobile.
As shown in FIG. 1, the master cylinder includes a first piston 3 and a second piston 4, which are arranged in a bore 2 of a cylinder body 1. The cylinder body 1 is formed with first and second oil inlets 6 in communication with an oil tank 5 and in turn, the first and second pistons 3 and 4 are formed respectively with communication holes 9 to introduce oil, having passed through the first and second oil inlets 6, into a first hydraulic pressure chamber 7 and a second hydraulic pressure chamber 8 defined in the bore 2 of the cylinder body 1. When the first and second pistons 3 and 4 are advanced, the communication holes 9 are displaced farther forward than sealing members 11, allowing the first and second hydraulic pressure chambers 7 and 8 to be pressed. On the contrary, when the first and second pistons 3 and 4 are retracted, the communication holes 9 are displaced farther rearward than the sealing members 11 until they are held in communication with the first and second oil inlets 6, allowing the oil in the first and second hydraulic pressure chambers 7 and 8 to be returned to the oil tank 5 and resulting in release of braking pressure.
Each of the sealing members 11, as shown in FIG. 2A, takes the form of a ring and is fitted in a receiving recess 1a formed in the cylinder body 1. The sealing member 11 is referred to as a “cup seal” because of a cup-shaped cross sectional shape thereof, and consists of an inner wing part 11a that comes into contact with an outer surface of the piston 4 and an outer wing part 11b that comes into contact with an inner surface of the receiving recess 1a. 
Referring to FIG. 2B, since an inner surface of the sealing member 11 is affected by pressure generated in the hydraulic pressure chamber 8 when the piston 4 is advanced, the inner and outer wing parts 11a and 11b are brought into close contact with the outer surface of the piston 4 and the inner surface of the receiving recess 1a, acting to intercept passage of the oil and consequently, causing a rise in the pressure of the hydraulic pressure chamber 8. The rising pressure of the hydraulic pressure chamber 8 accelerates deformation of the inner and outer wing parts 11a and 11b, resulting in closer contact of the sealing member 11.
However, the sealing member 11 tends to keep the deformed attitude thereof even when the piston 4 is retracted as shown in FIG. 2C. Therefore, the inner wing part 11a may close the communication hole 9 when the communication hole 9 reaches a tip end of the inner wing part 11a, thereby acting to intercept an oil return path. That is, since the deformed inner wing part 11a fails to be restored and thus, keeps the communication hole 9 closed even if the piston 4 is further retracted for a time after the communication hole 9 reaches the tip end of the inner wing part 11a, the piston 4 exhibits a so-called lost travel wherein the piston 4 is meaninglessly retracted by a predetermined distance upon release of braking, causing delay in a braking release operation. This phenomenon may be worsened when the pressure of the hydraulic pressure chamber 8 rises due to higher oil temperature in summer.